[PATCH v2] selftests/x86: Fix sysret_rip assertion failure on FRED systems

[Yi Lai <yi1.lai@intel.com>]

The existing 'sysret_rip' selftest asserts that 'regs->r11 ==
regs->flags'. This check relies on the behavior of the SYSCALL
instruction on legacy x86_64, which saves 'RFLAGS' into 'R11'.

However, on systems with FRED (Flexible Return and Event Delivery)
enabled, instead of using registers, all state is saved onto the stack.
Consequently, 'R11' retains its userspace value, causing the assertion
to fail.

Fix this by detecting if FRED is enabled and skipping the register
assertion in that case. The detection is done by checking if the RPL
bits of the GS selector are preserved after a hardware exception.
IDT (via IRET) clears the RPL bits of NULL selectors, while FRED (via
ERETU) preserves them.

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Yi Lai <yi1.lai@intel.com>
---
v2:
 - Replaced CPUID check with a runtime probe using INT3 and GS RPL
   preservation to robustly detect active FRED usage (Suggested by
   Andrew Cooper).

 tools/testing/selftests/x86/sysret_rip.c | 45 ++++++++++++++++++++++--
 1 file changed, 42 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/x86/sysret_rip.c b/tools/testing/selftests/x86/sysret_rip.c
index 2e423a335e1c..af63b04eccbe 100644
--- a/tools/testing/selftests/x86/sysret_rip.c
+++ b/tools/testing/selftests/x86/sysret_rip.c
@@ -33,6 +33,39 @@ extern const char test_page[];
 
 static const void *current_test_page_addr = test_page;
 
+static void empty_handler(int sig, siginfo_t *info, void *ctx_void)
+{
+}
+
+static bool is_fred_enabled(void)
+{
+	unsigned short gs_val;
+
+	sethandler(SIGTRAP, empty_handler, 0);
+
+	/*
+	 * Distinguish IDT and FRED mode by loading GS with a non-zero RPL and
+	 * triggering an exception:
+	 * IDT (IRET) clears RPL bits of NULL selectors.
+	 * FRED (ERETU) preserves them.
+	 *
+	 * If GS is loaded with 3 (Index=0, RPL=3), trigger an exception:
+	 * IDT should restore GS as 0.
+	 * FRED should preserve GS as 3.
+	 */
+	asm volatile (
+		"mov %[rpl3], %%gs\n\t"
+		"int3\n\t"
+		"mov %%gs, %[res]"
+		: [res] "=r" (gs_val)
+		: [rpl3] "r" (3)
+	);
+
+	clearhandler(SIGTRAP);
+
+	return gs_val == 3;
+}
+
 /* State used by our signal handlers. */
 static gregset_t initial_regs;
 
@@ -64,9 +97,15 @@ static void sigusr1(int sig, siginfo_t *info, void *ctx_void)
 	ctx->uc_mcontext.gregs[REG_RIP] = rip;
 	ctx->uc_mcontext.gregs[REG_RCX] = rip;
 
-	/* R11 and EFLAGS should already match. */
-	assert(ctx->uc_mcontext.gregs[REG_EFL] ==
-	       ctx->uc_mcontext.gregs[REG_R11]);
+	/*
+	 * SYSCALL works differently on FRED, it does not save RIP and RFLAGS
+	 * to RCX and R11.
+	 */
+	if (!is_fred_enabled()) {
+		/* R11 and EFLAGS should already match. */
+		assert(ctx->uc_mcontext.gregs[REG_EFL] ==
+		       ctx->uc_mcontext.gregs[REG_R11]);
+	}
 
 	sethandler(SIGSEGV, sigsegv_for_sigreturn_test, SA_RESETHAND);
 }
-- 
2.43.0